Method of forming an asymmetric membrane

ABSTRACT

The present disclosure provides methods for forming asymmetric membranes. More specifically, methods are provided for applying a polymerizable species to a porous substrate for forming a coated porous substrate. The coated porous substrate is exposed to an ultraviolet radiation source having a peak emission wavelength less than 340 nm to polymerize the polymerizable species forming a polymerized material retained within the porous substrate so that the concentration of polymerized material is greater at the first major surface than at the second major surface.

FIELD

The present disclosure relates to a method of forming an asymmetricmembrane.

BACKGROUND

Membranes can be used in separation processes where certain species areretained and other species are allowed to pass through the membrane.Some membrane applications include, for example, use in food andbeverage, pharmaceutical, medical, automotive, electronic, chemical,biotechnology, and dairy industries.

Asymmetric membranes have been described. Asymmetric membranes have beenformed with the addition of photoblockers and high photoinitiatorconcentrations under long wavelength ultraviolet radiation sources.

SUMMARY

The present disclosure provides methods of forming asymmetric membranes.

In one aspect, a method of forming an asymmetric membrane is provided.The method includes providing a porous substrate having a first majorsurface and a second major surface. The method includes applying apolymerizable composition to the porous substrate providing a coatedporous substrate. The polymerizable composition comprises at least onepolymerizable species and at least one photoinitiator. The methodincludes exposing the coated porous substrate to an ultravioletradiation source having a peak emission wavelength less than 340 nm topolymerize the polymerizable species providing an asymmetric membrane.The asymmetric membrane has a polymerized material retained within theporous substrate. The polymerized material has a concentration greaterat the first major surface than at the second major surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a porous substrate irradiatedwith a long wavelength ultraviolet radiation source.

FIG. 2 is a schematic representation of a porous substrate irradiatedwith an ultraviolet radiation source having a peak emission wavelengthless than 340 nm.

DETAILED DESCRIPTION

The recitation of numerical ranges by endpoints includes all numberssubsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.8, 4, and 5).

As included in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural referents unless the contentclearly dictates otherwise. Thus, for example, reference to acomposition containing “a compound” includes a mixture of two or morecompounds. As used in this specification and appended claims, the term“or” is generally employed in its sense including “and/or” unless thecontent clearly dictates otherwise.

Unless otherwise indicated, all numbers expressing quantities oringredients, measurement of properties and so forth used in thespecification and claims are to be understood as being modified in allinstances by the term “about.”

Ultraviolet radiation sources are effective for initiating andpolymerizing polymerizable compositions to provide for polymerizedmaterial retained within a porous substrate. The polymerizable speciesof the polymerizable composition can polymerize within the pores of theporous substrate. A gradient concentration of polymerized material canbe retained throughout at least a portion of the thickness of the poroussubstrate providing for an asymmetric membrane. In some embodiments, alow wavelength ultraviolet radiation source can be selected fordelivering radiation to a coated porous substrate. The irradiancedelivered to a first major surface is greater than the irradiancedelivered at the second major surface. The irradiance can decrease asthe radiation travels and is absorbed progressing through the thicknessof the coated porous substrate. During exposure to the ultravioletradiation source, the polymerizable composition located at the firstmajor surface can receive a greater irradiance than the polymerizablecomposition at the second major surface.

The method of the present disclosure provides for a continuous processfor forming high flux asymmetrical membranes relative to symmetricalmembranes of the same composition. The term “asymmetric” refers to amembrane in which the pore size and structure are not the same from oneside of the membrane to the other side. The pores of the asymmetricmembranes are partially filled (e.g., gel-filled) with polymerizedmaterial. Irradiating one side of the coated porous substrate with anultraviolet radiation source having a peak emission wavelength less than340 nm under an oxygen (O₂) free environment can result in anasymmetrical distribution of polymerized material retained within theporous substrate. The process can be accomplished without the additionof 1) high concentrations of photoinitiator and/or 2) photoblockers, andwithout the application of long wavelength radiation sources. Forexample, the asymmetric membranes formed herein have high flux and goodsalt rejections in water softening applications.

Porous substrates are materials having a network of interconnectingpassages extending from one surface to the other. These interconnectingpassages provide tortuous passageways through which liquids beingfiltered must pass.

In the method of the present disclosure, a porous substrate having afirst major surface, pores (e.g., interstitial), and a second majorsurface can be selected from a variety of materials so long as theporous substrate is coatable (e.g., capable of having a polymerizablecomposition applied to at least a portion of the thickness of thesubstrate) or can be adapted to be coatable, and comprises openings orpores. The first major surface of the porous substrate refers to thesurface in close proximity to the ultraviolet radiation source. Thesecond major surface, or an opposing surface to the first major surface,is located at a distance greater to the ultraviolet radiation sourcethan the distance of the first major surface to the ultravioletradiation source.

Suitable porous substrates include, for example, films, porousmembranes, woven webs, nonwoven webs, hollow fibers, and the like Theporous substrate can be formed from polymeric materials, ceramicmaterials, and the like, or combinations thereof. Some suitablepolymeric materials include, for example, polyolefins, poly(isoprenes),poly(butadienes), fluorinated polymers, polyvinyl chlorides, polyesters,polyamides, polyimides, polyethers, poly(ether sulfones),poly(sulfones), poly(ether)sulfones, polyphenylene oxides, polyphenylenesulfides, poly(vinyl acetates), copolymers of vinyl acetate, poly(phosphazenes), poly(vinyl esters), poly(vinyl ethers), poly(vinylalcohols), poly(carbonates) and the like, or combinations thereof.Suitable polyolefins include, for example, poly(ethylene),poly(propylene), poly(1-butene), copolymers of ethylene and propylene,alpha olefin copolymers (such as copolymers of 1-butene, 1-hexene,1-octene, and 1-decene), poly(ethylene-co-1-butene),poly(ethylene-co-1-butene-co-1-hexene), and the like, or combinationsthereof. Suitable fluorinated polymers include, for example, poly(vinylfluoride), poly(vinylidene fluoride), copolymers of vinylidene fluoride(such as poly(vinylidene fluoride-co-hexafluoropropylene)), copolymersof chlorotrifluoroethylene (such aspoly(ethylene-co-chlorotrifluoroethylene)), and the like, orcombinations thereof. Suitable polyamides include, for example,poly(imino(1-oxohexamethylene)), poly(iminoadipoylimino hexamethylene),poly(iminoadipoyliminodecamethylene), polycaprolactam, and the like, orcombinations thereof. Suitable polyimides include, for example,poly(pyromellitimide), and the like. Suitable poly(ether sulfone)sinclude, for example, poly(diphenylether sulfone),poly(diphenylsulfone-co-diphenylene oxide sulfone), and the like, orcombinations thereof.

In some embodiments, the porous substrate can have an average pore sizeless than about 10 micrometers. In other embodiments, the average poresize of the porous substrate can be less than about 5 micrometers, lessthan about 2 micrometers, or less than about 1 micrometer. In otherembodiments, the average pore size of the porous substrate can begreater than about 10 nanometers. In some embodiments, the average poresize of the porous substrate is greater than about 50 nanometers,greater than about 100 nanometers, or greater than about 200 nanometers.In some embodiments, the porous substrate can have an average pore sizein a range of about 10 nanometers to about 10 micrometers, in a range ofabout 50 nanometers to about 5 micrometers, in a range of about 100nanometers to about 2 micrometers, or in a range of about 200 nanometersto about 1 micrometer.

Some suitable porous substrates include, for example, nanoporousmembranes, microporous membranes, microporous nonwoven webs, microporouswoven webs, microporous fibers, and the like. In some embodiments, theporous substrate can have a combination of different pore sizes (e.g.,micropores, nanopores, and the like). In one embodiment, the poroussubstrate is microporous. In some embodiments, the porous substrate cancomprise a particulate or a plurality of particulates.

The thickness of the porous substrate selected can depend on theintended application of the membrane. Generally, the thickness of theporous substrate can be greater than about 10 micrometers. In someembodiments, the thickness of the porous substrate can be greater thanabout 1,000 micrometers, or greater than about 10,000 micrometers.

In some embodiments, the porous substrate is hydrophobic. In anotherembodiment, the porous substrate is hydrophilic. The porous substrateeither being hydrophobic or hydrophilic can be coated with apolymerizable composition and exposed to an ultraviolet radiation sourceas described below.

In some embodiments, the porous substrate comprises a microporous,thermally-induced phase separation (TIPS) membrane. TIPS membranes canbe prepared by forming a solution of a thermoplastic material and asecond material above the melting point of the thermoplastic material.Upon cooling, the thermoplastic material crystallizes and phaseseparates from the second material. The crystallized material can bestretched. The second material can be optionally removed either beforeor after stretching. TIPS membranes are disclosed in U.S. Pat. No.1,529,256 (Kelley); U.S. Pat. No. 4,726,989 (Mrozinski); U.S. Pat. No.4,867,881 (Kinzer); U.S. Pat. No. 5,120,594 (Mrozinski); U.S. Pat. No.5,260,360 (Mrozinski); U.S. Pat. No. 5,962,544 (Waller, Jr.); and U.S.Pat. No. 4,539,256 (Shipman). In some embodiments, TIPS membranescomprise polymeric materials such as poly(vinylidene fluoride) (i.e.,PVDF), polyolefins such as poly(ethylene) or poly(propylene),vinyl-containing polymers or copolymers such as ethylene-vinyl alcoholcopolymers and butadiene-containing polymers or copolymers, andacrylate-containing polymers or copolymers. TIPS membranes comprisingPVDF are further described in U.S. Patent Application Publication No.2005/0058821 (Smith et al.)

In some embodiments, the porous substrate can be a nonwoven web havingan average pore size that is typically greater than about 10micrometers. Suitable nonwoven webs include, for example, melt-blownmicrofiber nonwoven webs described in Wente, V. A., “SuperfineThermoplastic Fibers”; Industrial Engineering Chemistry, 48, 1342-1346(1956), and Wente, V. A., “Manufacture of Super Fine Organic Fibers”;Naval Research Laboratories (Report No. 4364) May 25, 1954. In someembodiments, suitable nonwoven webs can be prepared from nylon.

Some examples of suitable porous substrates include commerciallyavailable materials such as hydrophilic and hydrophobic microporousmembranes known under the trade designations DURAPORE and MILLIPOREEXPRESS MEMBRANE, available from Millipore Corporation of Billerica,Mass. Other suitable commercial microporous membranes known under thetrade designations NYLAFLO and SUPOR are available from Pall Corporationof East Hills, N.Y.

In the method of the present disclosure, a polymerizable species isapplied to the porous substrate. The term “polymerizable composition”generally refers to compositions having at least one polymerizablespecies, and at least one photoinitiator. The polymerizable species canbe polymerized on the first major surface, within the pores or at leasta portion of the pores, or on the second major surface of the poroussubstrate when exposed to an ultraviolet radiation source having a peakemission wavelength of less than 340 nm. The photoinitator selected forinitiating the polymerization of the polymeric species can selectivelyabsorb radiation from the ultraviolet radiation sources. In someembodiments, the polymerizable composition applied to the poroussubstrate doesn't require a photoinitiator as described in U.S. Pat. No.5,891,530 (Wright). The polymerizable composition can be applied to atleast a portion of the thickness of the porous substrate. Thepolymerizable species of the polymerizable composition, after exposureto the ultraviolet radiation source, can form polymerized materialextending through at least a portion of the thickness of the poroussubstrate. The resulting polymerized material can reside on the firstmajor surface, the second major surface, and within the porous substrateby chemical or physical interactions. In some embodiments, thepolymerized material can graft onto the surfaces of the poroussubstrate. In another embodiment, the polymerized material can residewithin and on the surfaces of the pores of the porous substrate throughhydrogen bonding, Van der Waals interactions, ionic bonding, and thelike.

The photoinitiator of the polymerizable composition can initiatepolymerization of the polymerizable species. The polymerizablecomposition can comprise about 0.001 to about 5.0 weight percentphotoinitiator. Some suitable photoinitiators can include, for example,organic compounds, organometallic compounds, inorganic compounds, andthe like. Some examples of free radical photoinitiators include, forexample, benzoin and its derivatives, benzyl ketals, acetophenone,acetophenone derivatives, benzophenone, and benzophenone derivatives,acyl phosphine oxides, and the like, or combinations thereof. In someembodiments, some photoinitiators (e.g., acyl phosphine oxides) canabsorb long wavelength ultraviolet radiation, short wavelengthultraviolet radiation, and the like or combinations thereof.

Exemplary photoinitiators for initiating free-radical polymerization of(meth)acrylates, for example, include benzoin and its derivatives suchas alpha-methylbenzoin; alpha-phenylbenzoin; alpha-allylbenzoin;alpha-benzylbenzoin; benzoin ethers such as benzil dimethyl ketal(available, for example, under the trade designation IRGACURE 651 fromCiba Specialty Chemicals, Tarrytown, N.Y.), benzoin methyl ether,benzoin ethyl ether, benzoin n-butyl ether; acetophenone and itsderivatives such as 2-hydroxy-2-methyl-1-phenyl-1-propanone (available,for example, under the trade designation DAROCUR 1173 from CibaSpecialty Chemicals) and 1-hydroxycyclohexyl phenyl ketone (available,for example, under the trade designation IRGACURE 184 from CibaSpecialty Chemicals);2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone(available, for example, under the trade designation IRGACURE 907 fromCiba Specialty Chemicals);2-benzyl-2-(dimethlamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone(available, for example, as IRGACURE 369 from Ciba Specialty Chemicals).Other useful photoinitiators include pivaloin ethyl ether, anisoin ethylether; anthraquinones, such as anthraquinone, 2-ethylanthraquinone,1-chloroanthraquinone, 1,4-dimethylanthraquinone,1-methoxyanthraquinone, benzanthraquinonehalomethyltriazines;benzophenone and its derivatives; iodonium salts and sulfonium salts asdescribed hereinabove; titanium complexes such asbis(eta₅-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium(obtained under the trade designation CGI 784 DC, also from CibaSpecialty Chemicals); halomethylnitrobenzenes such as, for example,4-bromomethylnitrobenzene; mono- and bis-acylphosphines (available, forexample, from Ciba Specialty Chemicals as IRGACURE 1700, IRGACURE 1800,IRGACURE 1850, and DAROCUR 4265).

The photoiniator of the polymerizable composition can be selected toinitiate polymerization of the polymerizable species throughout at leasta portion of the thickness of the porous substrate. The thickness of theporous substrate extends from the first major surface to the secondmajor surface. The photoinitiator can initiate polymerization of thepolymerizable species upon exposure to the ultraviolet radiation sourceat the first major surface, and can extend through a portion of thethickness of the porous substrate. The initiation of polymerizablespecies for forming polymerized material can decrease through thethickness to the second major surface.

Polymerizable species (e.g., monomers) of the polymerizable compositioncan polymerize by many polymerization routes. In particular, thepolymerizable species can attach to another polymerizable species bychemical bonding (e.g., free radical reaction) to form a covalent bondthrough known polymerization procedures. Upon polymerizing thepolymerizable species of the coated porous substrate when contacted withan ultraviolet radiation source can form an asymmetric membrane. Thesurface properties of the porous substrate before being coated with thepolymerizable composition can be different than the surface propertiesof the asymmetric membrane described herein. Similarly, the asymmetricmembrane having functional groups can have different major surfaceproperties than that of the porous substrate. For example, the additionof polymerized material to the porous substrate can provide for reactivesurfaces when contacted by other species, for example, by interactionsincluding hydrogen bonding, Van der Waals interactions, ionic bonding,and the like.

In some embodiments, the polymerizable species of the polymerizablecomposition can be a monomer having a free-radically polymerizablegroup. In some embodiments, the polymerizable species may comprise afree-radically polymerizable group and an additional functional groupthereon. The free-radically polymerizable group can be an ethylenicallyunsaturated group such as a (meth)acryloyl group, an acryoyl group, or avinyl group. The free-radically polymerizable group, after initiation bya photoinitiator, can polymerize within the porous substrate forming apolymerized material upon exposure to the ultraviolet radiation source.The reaction of the free-radically polymerizable groups of thepolymerizable species with other polymerizable species of the coatedporous substrate upon exposure to ultraviolet radiation can result inthe formation of a greater concentration of the polymerized material atthe first major surface and within the openings or pores nearest thefirst major surface than at the second major surface of the asymmetricmembrane.

In addition to having a free-radically polymerizable group,polymerizable species can contain a second or additional functionalgroup. In some embodiments, the second functional group is selected froma second ethylenically unsaturated group, ring opening groups (e.g.,epoxy group, an azlactone group, and an aziridine group), an isocyanatogroup, an ionic group, an alkylene oxide group, or combinations thereof.The second or additional functional group of the polymerizable speciescan provide for further reactivity or affinity of the polymerizedmaterial retained within the porous substrate. In some embodiments, theadditional functional group can react to form a linking group betweenthe porous substrate and other material such as other species ornucleophilic compounds having at least one nucleophilic group.

The presence of an additional functional group can impart a desiredsurface property to the asymmetric membrane such as an affinity for aparticular type of compound. In some embodiments, the polymerizablespecies can contains an ionic group such that the asymmetric membranecontaining polymerized material can often have an affinity for compoundshaving an opposite charge. That is, compounds with negatively chargedgroups can be attracted to an asymmetric membrane having polymerizedmaterial with a cationic group and compounds with positively chargedgroups can be attracted to a an asymmetric membrane having polymerizedmaterial with an anionic group. Further, the choice of polymerizedmaterial can impart a hydrophilic property to at least one major surfaceof the asymmetric membrane that was hydrophobic prior to surfacemodification by the polymerizable composition. In one embodiment, thepolymerized material containing an alkylene oxide group can imparthydrophilic character to the asymmetric membrane.

In still other embodiments, suitable polymerizable species of thepolymerizable composition can have a free-radically polymerizable groupthat is an ethylenically unsaturated group and an additional functionalgroup that is an ionic group. The ionic group can have a positivecharge, a negative charge, or a combination thereof. With some suitableionic species, the ionic group can be neutral or charged depending onthe pH conditions. This class of species is typically used to impart adesired surface affinity for one or more oppositely charged compounds orto decrease the affinity for one or more similarly charged compounds.

In still other embodiments, suitable ionic polymerizable species havinga negative charge include (meth)acrylamidosulfonic acids of Formula I orsalts thereof

In Formula I, R¹ is hydrogen or methyl; and Y is a straight or branchedalkylene (e.g., alkylenes having 1 to 10 carbon atoms, 1 to 6 carbonatoms, or 1 to 4 carbon atoms). Exemplary ionic species according toFormula I include, but are not limited to, N-acrylamidomethanesulfonicacid, 2-acrylamidoethanesulfonic acid,2-acrylamido-2-methyl-1-propanesulfonic acid, and2-methacrylamido-2-methyl-1-propanesulfonic acid. Salts of these acidicspecies can also be used. Counter ions for the salts can be, forexample, ammonium ions, potassium ions, lithium ions, or sodium ions.

Other suitable ionic polymerizable species having a negative chargeinclude sulfonic acids such as vinylsulfonic acid and 4-styrenesulfonicacid; (meth)acrylamidophosphonic acids such as(meth)acrylamidoalkylphosphonic acids (e.g., 2-acrylamidoethylphosphonicacid and 3-methacrylamidopropylphosphonic acid); acrylic acid andmethacrylic acid; and carboxyalkyl(meth)acrylates such as2-carboxyethylacrylate, 2-carboxyethylmethacrylate,3-carboxypropylacrylate, and 3-carboxypropylmethacrylate. Still othersuitable acidic species include (meth)acryloylamino as described in U.S.Pat. No. 4,157,418 (Heilmann et al). Exemplary (meth)acryloylamino acidsinclude, but are not limited to, N-acryloylglycine, N-acryloylasparticacid, N-acryloyl-β-alanine, and 2-acrylamidoglycolic acid. Salts of anyof these acidic species can also be used.

Other ionic polymerizable species that are capable of providing apositive charge are amino (meth)acrylates or amino (meth)acrylamides ofFormula II or quaternary ammonium salts thereof. The counter ions of thequaternary ammonium salts are often halides, sulfates, phosphates,nitrates, and the like.

In Formula II, R¹ is hydrogen or methyl; L is oxy or —NH—; and Y is analkylene (e.g., an alkylene having 1 to 10 carbon atoms, 1 to 6, or 1 to4 carbon atoms). Each R² is independently hydrogen, alkyl, hydroxyalkyl(i.e., an alkyl substituted with a hydroxy), or aminoalkyl (i.e., analkyl substituted with an amino). Alternatively, the two R² groups takentogether with the nitrogen atom to which they are attached can form aheterocyclic group that is aromatic, partially unsaturated (i.e.,unsaturated but not aromatic), or saturated, wherein the heterocyclicgroup can optionally be fused to a second ring that is aromatic (e.g.,benzene), partially unsaturated (e.g., cyclohexene), or saturated (e.g.,cyclohexane).

In some embodiments of Formula II, both R² groups are hydrogen. In otherembodiments, one R² group is hydrogen and the other is an alkyl having 1to 10, 1 to 6, or 1 to 4 carbon atoms. In still other embodiments, atleast one of R² groups is a hydroxy alkyl or an amino alkyl that have 1to 10, 1 to 6, or 1 to 4 carbon atoms with the hydroxy or amino groupbeing positioned on any of the carbon atoms of the alkyl group. In yetother embodiments, the R² groups combine with the nitrogen atom to whichthey are attached to form a heterocyclic group. The heterocyclic groupincludes at least one nitrogen atom and can contain other heteroatomssuch as oxygen or sulfur. Exemplary heterocyclic groups include, but arenot limited to imidazolyl. The heterocyclic group can be fused to anadditional ring such as a benzene, cyclohexene, or cyclohexane.Exemplary heterocyclic groups fused to an additional ring include, butare not limited to, benzoimidazolyl.

Exemplary amino (meth)acrylates (i.e., L in Formula II is oxy) include,for example, N,N-dialkylaminoalkyl(meth)acrylates such as, for example,N,N-dimethylaminoethylmethacrylate, N,N-dimethylaminoethylacrylate,N,N-diethylaminoethylmethacylate, N,N-diethylaminoethylacrylate,N,N-dimethylaminopropylmethacrylate, N,N-dimethylaminopropylacrylate,N-tert-butylaminopropylmethacrylate, N-tert-butylaminopropylacrylate andthe like.

Exemplary amino (meth)acrylamides (i.e., L in Formula II is —NH—)include, for example, N-(3-aminopropyl)methacrylamide,N-(3-aminopropyl)acrylamide, N-[3-(dimethylamino)propyl]methacrylamide,N-(3-imidazolylpropyl)methacrylamide, N-(3-imidazolylpropyl)acrylamide,N-(2-imidazolylethyl)methacrylamide,N-(1,1-dimethyl-3-imidazoylpropyl)methacrylamide,N-(1,1-dimethyl-3-imidazoylpropyl)acrylamide,N-(3-benzoimidazolylpropyl)acrylamide, andN-(3-benzoimidazolylpropyl)methacrylamide.

Exemplary quaternary salts of the ionic species of Formula II include,but are not limited to, (meth)acrylamidoalkyltrimethylammonium salts(e.g., 3-methacrylamidopropyltrimethylammonium chloride and3-acrylamidopropyltrimethylammonium chloride) and(meth)acryloxyalkyltrimethylammonium salts (e.g.,2-acryloxyethyltrimethylammonium chloride,2-methacryloxyethyltrimethylammonium chloride,3-methacryloxy-2-hydroxypropyltrimethylammonium chloride,3-acryloxy-2-hydroxypropyltrimethylammonium chloride, and2-acryloxyethyltrimethylammonium methyl sulfate).

Other polymerizable species can be selected from those known to providepositively charged groups, for example, to an ion exchange resin. Suchpolymerizable species include, for example, the dialkylaminoalkylamineadducts of alkenylazlactones (e.g., 2-(diethylamino)ethylamine,(2-aminoethyl)trimethylammonium chloride, and3-(dimethylamino)propylamine adducts of vinyldimethylazlactone) anddiallylamine species (e.g., diallylammonium chloride anddiallyldimethylammonium chloride).

In some methods for making an asymmetric membrane, suitablepolymerizable species can have two free-radically polymerizable groupsas well as a hydrophilic group. For example, alkylene glycoldi(meth)acrylates can be used as polymerizable species to imparthydrophilic character to a hydrophobic porous substrate. Thesepolymerizable species have two (meth)acryloyl groups and a hydrophilicpolyalkylene glycol (i.e., polyalkylene oxide) group.

When the membrane has polymerizable species that contains an epoxygroup, an azlactone group, or an isocyanato group, the asymmetricmembrane can be further treated such that the functional groups canreact with a nucleophilic compound having a one or a plurality ofnucleophilic groups to impart a hydrophilic character to a hydrophobicporous substrate. Unreacted nucleophilic groups can contribute toforming a hydrophilic functionalized membrane. Some exemplarynucleophilic compounds contain a hydrophilic group such as apolyalkylene oxide group in addition to the nucleophilic group. Forexample, the nucleophilic compound such as polyalkylene glycol diaminesand polyalkylene glycol triamines can include a plurality of aminogroups.

Polymerizable compositions of the present disclosure can be prepared,for example, as a coatable solution, dispersion, emulsion, and the like.The polymerizable compositions can be applied to the first majorsurface, interstitial pores, and the second major surface of the poroussubstrate. In some examples, the porous substrate can be saturated orimmersed with a polymerizable composition comprising at least onepolymerizable species and at least one photoinitiator effective forcoating the first major surface, interstitial pores and the second majorsurface. The concentration of the polymerizable species, for example,can vary depending on a number of factors including, but not limited to,the polymerizable species, the extent of polymerization or crosslinkingof the polymerizable species on and within the porous substrate, thereactivity of the polymerizable species, the crosslinker concentration,or the solvent used. In some embodiments, the concentration of thepolymerizable species of the polymerizable composition can be in a rangeof about 2 weight percent to about 99.9 weight percent.

In some embodiments, the polymerizable composition further comprises asolvent. In one aspect, the polymerizable composition further comprisesa crosslinker.

In one embodiment, the porous substrate can have a hydrophilic surfaceprior to contacting the polymerizable composition. After contacting thepolymerizable composition with an ultraviolet radiation source having apeak emission wavelength less than 340 nm, the hydrophobic surface canimpart a hydrophobic property to at least one surface of the asymmetricmembrane.

In some embodiments, the polymerizable species of the polymerizablecomposition have a free-radically polymerizable group that is a firstethylenically unsaturated group and a second functional group that is asecond ethylenically unsaturated group. In one embodiment, thepolymerizable species is a crosslinker suitable for crosslinking thepolymerizable species forming a network or gelled polymerized material.Suitable polymerizable species having two ethylenically unsaturatedgroups include, but are not limited to, polyalkylene glycoldi(meth)acrylates. The term polyalkylene glycol di(meth)acrylate is usedinterchangeably with the term polyalkylene oxide di(meth)acrylate. Theterm “(meth)acryl” as in (meth)acrylate is used to encompass both acrylgroups as in acrylates and methacryl groups as in methacrylates.Exemplary polyalkylene glycol di(meth)acrylates include polyethyleneglycol di(meth)acrylate species and polypropylene glycoldi(meth)acrylate species. Polyethylene glycol diacrylate species havingan average molecular weight of about 400 g/mole is commerciallyavailable, for example, under the trade designation SR344 andpolyethylene glycol dimethacrylate species having an average molecularweight of about 400 g/mole is commercially available under the tradedesignation SR603 from Sartomer Company, Incorporated of Exton, Pa.

In some embodiments, suitable polymerizable species have afree-radically polymerizable group that is a first ethylenicallyunsaturated group and an additional functional group that is an epoxygroup. Suitable polymerizable species within this class include, but arenot limited to, glycidyl (meth)acrylates. This class of polymerizablespecies can provide a functionalized asymmetric membrane having at leastone epoxy group available for further reactivity. The epoxy group canreact with other reactants such as with another species or with anucleophilic compound to impart a desired surface property to the poroussubstrate (e.g., affinity for a particular compound or functional grouphaving different reactivity). The reaction of the epoxy group with anucleophilic compound, for example, results in the opening of the epoxyring and the formation of a linkage group that functions to tether thenucleophilic compound to the porous substrate. Suitable nucleophilicgroups for reacting with epoxy groups include, but are not limited to,primary amino groups, secondary amino groups, and carboxy groups. Thenucleophilic compound can contain more than one nucleophilic group thatcan crosslink multiple epoxy groups or more than one optional groupsthat can impart hydrophilic character to the functionalized membrane.The linkage group formed by ring-opening of the epoxy group oftencontains the group —C(OH)HCH₂NH— when the epoxy is reacted with aprimary amino group or —C(OH)HCH₂O(CO)— when the epoxy is reacted with acarboxy group.

In some instances, the epoxy groups of the polymerized material withinthe porous substrate can be reacted with a multifunctional amine such asa diamine having two primary amino groups or a triamine having threeprimary amino groups. One of the amino groups can undergo a ring openingreaction with the epoxy group and result in the formation of a linkagegroup that contains the group —C(OH)HCH₂NH-between the nucleophiliccompound and the porous substrate. The second amino group or the secondand third amino groups can impart a hydrophilic character to theasymmetric membrane or can crosslink two or more polymerizable speciesby reacting with one or more additional epoxy groups. In some examples,the multifunctional amine is a polyalkylene glycol diamine orpolyalkylene glycol triamine and reaction with an epoxy group results inthe attachment of a polymerized material having a polyalkylene glycolgroup (i.e., polyalkylene oxide group). The polyalkylene glycol group aswell as any terminal primary amino group tends to impart hydrophiliccharacter to the asymmetric membrane.

In still other embodiments, suitable polymerizable species have afree-radically polymerizable group that is an ethylenically unsaturatedgroup and an additional functional group that is an azlactone group.Suitable polymerizable species include, but are not limited to, vinylazlactone such as 2-vinyl-4,4-dimethylazlactone. This class ofpolymerizable species can provide an asymmetric membrane having at leastone azlactone group available for further reactivity. The azlactonegroup can react with other reactants such as another species or with anucleophilic compound to impart a desired surface property to the poroussubstrate (e.g., affinity for a particular compound or functional grouphaving different reactivity). The reaction of the azlactone group with anucleophilic compound, for example, results in the opening of theazlactone ring and the formation of a linkage group that functions toattach the nucleophilic compound to the porous substrate. Thenucleophilic compound typically contains at least one nucleophilicgroup. Suitable nucleophilic groups for reacting with an azlactone groupinclude, but are not limited to, primary amino groups, secondary aminogroups and hydroxy groups. The nucleophilic compound can containadditional nucleophilic groups that can crosslink multiple azlactonegroups or can contain other optional groups that can impart ahydrophilic character to the asymmetric membrane. The linkage groupformed by ring-opening of the azlactone group often contains the group—(CO)NHCR₂(CO)— where R is an alkyl such as methyl and (CO) denotes acarbonyl.

In some instances, the azlactone groups can be reacted with amultifunctional amine such as a diamine having two primary amino groupsor a triamine having three primary amino groups.

One of the amino groups can undergo a ring opening reaction with theazlactone group and result in the formation of a linkage containing thegroup —(CO)NHCR₂(CO)— between the nucleophilic compound and the poroussubstrate. The second amino group or second and third amino groups canimpart a hydrophilic character to the asymmetric membrane or cancrosslink multiple polymerizable species. In some examples, themultifunctional amine is a polyalkylene glycol diamine or a polyalkyleneglycol triamine and reaction with an azlactone group results in theattachment of a polymerizable species having a polyalkylene glycol group(i.e., polyalkylene oxide group). The polyalkylene glycol group as wellas any terminal primary amino group tends to impart a hydrophiliccharacter to the asymmetric membrane.

In still other embodiments, suitable polymerizable species can have afree-radically polymerizable group that is an ethylenically unsaturatedgroup and an additional functional group that is an isocyanato group.Some suitable polymerizable species include, but are not limited to anisocyanatoalkyl (meth)acrylate such as 2-isocyanatoethyl methacrylateand 2-isocyanatoethyl acrylate. This class of polymerizable species canprovide an asymmetric membrane having at least one isocyanato groupavailable for reactivity. The isocyanato group can react with otherreactants such as another species or with a nucleophilic compound toimpart a desired surface property to the asymmetric membrane (e.g.,affinity for a particular compound or functional group having differentreactivity). The reaction of an isocyanato group with a nucleophiliccompound can result in the formation of a urea linkage if thenucleophilic group is a primary amino or secondary amino group or in theformation of a urethane linkage if the nucleophilic group is a hydroxygroup. The nucleophilic compound can contain additional nucleophilicgroups that can crosslink multiple isocyanato groups or can containother optional groups that can impart a hydrophilic character to theasymmetric membrane. The linkage group formed by reaction of anucleophilic compound with an isocyanato group often contains the group—NH(CO)NH— when the nucleophilic group is a primary amino group or—NH(CO)O— when the nucleophilic group is a hydroxy.

In some embodiments, the polymerizable species can comprise unreactivependent groups. The polymerized material can be retained within thepores due to physical entanglements.

In some embodiments, the polymerizable composition further comprises acrosslinker to gel or form a network of polymerized material within theporous substrate. The crosslinker can be added to crosslink a portion ofthe polymerizable species, or to substantially crosslink most or all ofthe polymerizable species.

Crosslinkers (e.g., crosslinking materials) of the polymerizablecomposition can include difunctional and polyfunctional acrylate andmethacrylate free radically polymerizable monomers. Some examples ofcrosslinkers can include, for example, ester derivatives of alkyl diols,triols, and tetrols (e.g., 1,4-butanediol diacrylate, 1,6-hexanedioldiacrylate, trimethylolpropane triacrylate, and pentaerythritoltriacrylate). Some other difunctional and polyfunctional acrylate andmethacrylate monomers have been described in U.S. Pat. No. 4,379,201(Heilmann et al.). In some embodiments, difunctional and polyfunctionalacrylate monomers include, for example 1,2-ethanediol diacrylate,1,12-dodecanediol diacrylate, pentaerythritol tetracrylate, and thelike, or combinations thereof. Difunctional and polyfunctional acrylatesand methacrylates can include acrylated epoxy oligomers, acrylatedaliphatic urethane oligomers, acrylated polyether oligomers, andacrylated polyester oligomers such as those commercially available underthe trade designation EBECRYL from CYTEC SURFACE SPECIALTIES of Smyrna,Ga. Examples of other commercially available monomers as described aboveare available from Sartomer of Exton, Pa.

The polymerizable composition is applied to the porous substrate so asto coat, soak, wet, or immerse the porous substrate to provide a coatedporous substrate. The polymerizable composition can be applied to theporous substrate having a thickness extending from a first major surfaceto a second major surface of the porous substrate. The polymerizablecomposition can be applied to the porous substrate to wet or penetrateinto at least one micrometer of the thickness extending from the firstmajor surface. In some embodiments, the polymerizable composition canwet or penetrate through the entire thickness of the porous substrate.In some embodiments, the porous substrate can be immersed with thepolymerizable species. “Immersed” or “saturated” generally refers to thepolymerizable composition being delivered to the first major surface, tothe interconnecting pores within the thickness of the porous substrate,and to the second major surface. In some embodiments, the polymerizablecomposition can wet the surfaces of the pores throughout the thicknessof the porous substrate to include wetting the first and second majorsurfaces. Suitable methods for applying the polymerizable species to theporous substrate include, for example, saturation or immersiontechniques, spray coating, curtain coating, slide coating, floodcoating, die coating, roll coating, deposition, or by other knowncoating or application methods. The polymerizable composition to beapplied to the porous substrate generally has a viscosity such that thefirst major surface, the second major surface and the pores of theporous substrate can be coated. The viscosity of the polymerizablespecies can be altered dependent on the application method chosen toreceive the polymerizable composition.

After the polymerizable composition has been applied to the poroussubstrate, the coated porous substrate can be exposed to an ultravioletradiation source having a peak emission wavelength less than 340 nm toinitiate and polymerize the polymerizable species of the polymerizablecomposition. The ultraviolet radiation source selected for formingasymmetric membranes can depend on the intended processing conditions,and the appropriate energy source required for activating thephotoinitiator present in the polymerizable composition for providing agradient concentration of polymerized material through the thickness ofthe porous substrate. Similarly, other considerations for selecting theultraviolet radiation source can include the amount and type ofpolymerizable species, crosslinker, and related materials present in thepolymerizable composition, the ultraviolet radiation source used foractivating the photoinitiator to polymerizing the polymerizable species,the speed of the moving web (e.g., multilayer structure) for acontinuous process, the distance of the porous substrate from theultraviolet radiation source, and the thickness of the porous substrate.

A variety of ultraviolet (UV) radiation sources can be used to preparethe asymmetric membranes of the present disclosure. Suitable sourcesinclude low and medium-pressure mercury arc lamps, electrodeless mercurylamps, light emitting diodes, mercury-xenon lamps, lasers and any othersources having some spectral output in the region less than 340 nm.Available ultraviolet radiation sources can be broadband, narrowband ormonochromatic. When broadband ultraviolet radiation sources are used,filters can be applied to narrow the spectral output to a specificspectral region such that the peak intensity (e.g., emission) occurs ata wavelength less than 340 nm, thus eliminating longer wavelengths thatcan be detrimental to the rewettable asymmetric membrane formingprocess. Suitable ultraviolet radiation sources are not restricted bypower and can be pulsed or continuous sources. Some of these radiationsources may or may not contain mercury. Preferred ultraviolet radiationsources can be those that have relatively low IR (infrared) emissionsthat generally require no special cooling requirements. Dichroicreflectors (cold mirrors) and/or dichroic front windows (hot mirrors),and/or water jackets and other methods know to those skilled in the artcan be used to help control the IR emissions from the ultravioletradiation source.

In some embodiments, narrow bandwidth UV sources can be selected forwhich the UV radiation output spans a range of no more than about 50-100nm. One example of a narrow bandwidth UV radiation source includes, forexample, fluorescent ultraviolet lamps, which can operate withoutspecial filters and have low IR emissions. In a preferred embodiment,monochromatic or substantially monochromatic UV radiation sources suchas excimer lamps, lasers, light emitting diodes, and germicidal lampsare used. These sources have greater than 95% of their spectral outputconfined to a region spanning no more than about 20-30 nm. Some examplesof excimer lamps include a XeCl excimer lamp having a peak emission at308 nm, a KrCl excimer lamp having a peak emission at 222 nm, a Xe₂excimer lamp having a peak emission at 172 nm and a germicidal lamphaving a peak emission at 254 nm. Substantially monochromatic lampsproviding UV radiation output within a narrow spectral range and havinglow IR output are generally preferred. These lamps can allow for morecontrol in forming a gradient of polymerized material within thecopolymer retained within a rewettable asymmetric membrane and arecommercially available. Such sources are well known in the art. Anultraviolet radiation source can be a single source or a plurality ofsources. Similarly, the plurality of ultraviolet radiation sources canbe of the same source or of a combination of different ultravioletradiation sources.

Low and high power ultraviolet radiation sources (e.g., lamps) can beuseful for forming rewettable asymmetric membranes. Lamp power can beexpressed in watts/inch (W/in) based on the length of the lamp. Forexample, a high power lamp such as a 600 W/in electrodeless “H” bulb(Fusion UV Systems, Inc., Gaithersburg, Md.) is a 10-inch longmedium-pressure mercury bulb that can be excited by microwave energy. Atfull power, the 10 inch lamp requires a power supply rated at 6000 W todeliver power of 600 W/in. Such high power lamps can generate copiousamounts of UV radiation, but operate at lamp surface temperaturesexceeding 700° C. such that the ultraviolet output is accompanied bysignificant IR emissions. In contrast, a low power fluorescent UV lampcan operate at a typical power of 1-2 W/in, and requires less power tooperate having a surface temperature of about 43° C. to 49° C.

When exposing the coated porous substrate, the peak irradiance isgreater than 0 mW/cm² and can extend up to about 100 mW/cm² or greaterin the spectral region of the peak ultraviolet intensity and thespectral output must overlap with at least a portion of the absorptionspectrum of the photoinitiator.

The UV spectrum is split into four primary spectral regions known asUVA, UVB, UVC and VUV, which are commonly defined as 315-400 nm, 280-315nm, 200-280 nm and 100-200 nm, respectively. The wavelength ranges citedherein are somewhat arbitrarily established, and may not correspond tothe exact wavelength ranges published by radiometer manufacturers fordefining the four primary spectral regions. Furthermore, some radiometermanufacturers specify that a UVV range (395-445 nm) that spans thetransition from UV to visible radiation.

In some instances, high power UV radiation sources can be employed Thesesources can have a peak irradiance of more than about 1 W/cm²accompanied by significant IR emissions. More preferred ultravioletradiation sources can comprise an array of germicidal or fluorescentbulbs providing a peak UV irradiance in the range from about 1-2 μW/cm²to 10-20 mW/cm². The peak irradiance from an array or a plurality ofmicrowave-driven fluorescent lamps commercially available from QuantumTechnologies of Irvine, Calif., can be as high as 50 mW/cm². The actualirradiance from an array of lamps can depend on a number of factorswhich include the electrical voltage, the lamp's power rating, the lampspacing within an array or plurality of lamps, the reflector(s) type (ifpresent), the age of the individual lamps, the transmission spectrum ofany windows or films through which the UV radiation must pass, thespecific radiometer used and its spectral responsivity, and the distanceof the array of lamps from the membrane.

The porous substrate can be exposed to the ultraviolet radiation sourcefor a period of time (e.g., exposure time) for polymerizing thepolymerizable composition to form the asymmetric membrane. Some exposuretimes can range from less than a second at high irradiance (>1 W/cm²) toseveral seconds up to several minutes or longer at a low irradiance (<50mW/cm²). The total UV energy exposure to the porous substrate can bedetermined by the UV source irradiance and the exposure time. Forexample, an array of fluorescent or germicidal bulbs can be used toexpose the porous substrate to UV radiation. The total UV energy withinthe spectral range associated with the peak lamp output can be fromabout 100 mJ/cm² to more than about 4000 mJ/cm², from about 200 mJ/cm²to about 3000 mJ/cm², from about 300 mJ/cm² to about 2500 mJ/cm², orfrom about 400 mJ/cm² to about 2000 mJ/cm².

The rewettable asymmetric membrane of the present disclosure can beprepared such that a gradient concentration of polymerized materialextends from the first major surface through at least a portion of thethickness of the porous substrate to the second major surface. Uponexposure to the ultraviolet radiation source, the photoinitiatorresiding at the first major surface can be exposed to a greater peakirradiance of UV radiation. The higher peak irradiance at the firstmajor surface can result in a higher rate of initiation at the firstmajor surface and within the pores at the first major surface. As theirradiance travels into the thickness of the porous substrate, the peakirradiance decreases, thus reducing the amount of photoinitiatordecomposition and hence, polymerization within the pores of thesubstrate. A gradient concentration of polymerized material can beformed resulting from inner filter effects. The inner filter effects canoccur when certain wavelengths are selectively filtered out by absorbingspecies (e.g. photoinitiators, porous substrate, or combinationsthereof) as the ultraviolet radiation penetrates the thickness of theporous substrate. These wavelengths are effectively removed ordiminished. As the UV radiation penetrates further into or through theporous substrate, the wavelength distribution of the radiation impingingon the surface can be changed resulting from the absorption of certainwavelengths. At greater depths within the porous substrate, insufficientultraviolet radiation of the prescribed wavelength region can beavailable to efficiently excite the photoinitiator. The extent ofpolymerization of the polymerizable species can decrease rapidly forminga gradient concentration of polymerized material within the thickness ofthe porous substrate.

The sharpness of the gradient concentration of polymerized material canbe determined by the absorbance of the porous substrate at thewavelengths of the incident UV radiation. When sources other thansubstantially monochromatic sources are utilized, the absorbance isuncertain because absorbance is wavelength dependent. However, whensubstantially monochromatic sources are used, the absorbance(Beer-Lambert Law and measured using a UV-Visible spectrophotometer) atthe peak wavelength of the radiation source through a film of thepolymerizable composition at a thickness comparable to the membranethickness should be greater than 0.3, greater than 0.4, greater than 0.5or greater than 0.6. In some embodiments, the absorbance can be greaterthan 1.0 or even greater than 2.0 and as high as 10 or even 20.

The coated porous substrate selected for exposure to the ultravioletradiation source having a peak emission wavelength less than 340 nm canhave a thickness greater than about 10 micrometers. In some embodiments,the thickness of the coated porous substrate can be greater than about1,000 micrometers, or greater than about 10,000 micrometers. Thepolymerizable composition can saturate or immerse the porous substratesufficient for wetting at least a portion of the interconnected poresextending through the thickness from the first major surface to thesecond major surface.

The irradiance of ultraviolet radiation received by a coated poroussubstrate can affect the extent to which the polymerizable species arepolymerized. In some embodiments, at least 10 weight percent of thepolymerizable species can be polymerized. In other embodiments, at least20 weight percent, at least 30 weight percent, or at least 40 weightpercent of the polymerizable species can be polymerized form polymerizedmaterial residing within the thickness of the porous substrate.

The irradiance of the ultraviolet radiation delivered to the coatedporous substrate can be dependent upon, but not limited to, processingparameters including the type of ultraviolet radiation source selected,the line speed (e.g., continuous process line) used, and the distance ofthe ultraviolet radiation source to the first major surface of thecoated porous substrate. In some embodiments, the irradiance can beregulated by controlling the line speed. For example, at the irradiancedelivered to the first major surface can be greater at lower linespeeds, and the irradiance delivered to the first major surface at thefirst major surface can be reduced at faster line speeds.

The irradiance of the ultraviolet radiation source delivered to a coatedporous substrate can be dependent upon the residence time as describedabove. The extent of polymerization of the polymerizable speciesthroughout the thickness of the porous substrate can be controlled bythe irradiance and can affect the concentration of polymerized materialdistributed through the thickness of the coated porous substrate. Thepeak irradiance delivered through the thickness of the coated poroussubstrate can be, for example, in a range of greater than 0 to about 100mW/cm².

In some embodiments, the irradiance at the coated porous substrate uponexposure to the ultraviolet radiation source can be at least about 0.5micrometer extending into the thickness of the substrate from the firstmajor surface. In another embodiment, the irradiance delivered to thecoated porous substrate to polymerize the polymerizable species can beat least about 1 micrometer from the first major surface. In someembodiments, the irradiance delivered to the coated porous substrate canaffect the polymerizable species to at least about 2 micrometers, to atleast about 5 micrometers, to at least about 10 micrometers, or to atleast about 25 micrometers extending into the thickness of the poroussubstrate. While low irradiation and longer exposures are preferred forusing ultraviolet radiation sources, polymerizing polymerizable speciesas a matter of practical operation may necessitate speeds that canrequire higher irradiance and shorter exposures.

FIG. 1 illustrates the application of a long wavelength radiation source40 having a peak emission wavelength greater than 340 nm to a coatedporous substrate. FIG. 1 (comparative example) illustrates across-section of a porous substrate 5 irradiated by the long wavelengthradiation source 40. Porous substrate 5 comprises a first major surface10, a second major surface 20, and a pore 35. The long wavelengthradiation source 5 can irradiate the polymerizable species to formpolymerized material 30 which extends through the thickness of theporous substrate from the first major surface 10 to the second majorsurface 20. After irradiance of the porous substrate 5, a symmetricmembrane can be formed.

FIG. 2 illustrates the application of an ultraviolet radiation source 80having a peak emission wavelength less than 340 nm to a coated poroussubstrate is illustrated in FIG. 2. FIG. 2 illustrates a cross-sectionof a porous substrate 45 irradiated by a low intensity radiation source80. Porous substrate comprises a first major surface 50, a second majorsurface 60, and a pore 95. The ultraviolet radiation source 80irradiates the polymerizable species forming a first concentration ofpolymerized material 65 (first polymerized material) which extendsthrough a portion of the pore 95 in proximity of the first major surface50 extending to an average polymerized material concentration location85. In pore 95, a portion of pore 95 can contain a second concentrationof polymerized material 70 (second polymerized material) in proximity ofthe second major surface 60. The first concentration and the secondconcentrations of polymerized material recited in FIG. 2 are merely forillustrative purposes, and do not define absolute concentrations ofpolymerized materials retained within the pores of the porous substrate.A first polymerized material-pore interface 90 is formed where the firstconcentration of polymerized material 65 contacts the pore 95. A secondpolymerized material-pore interface 100 is formed where the secondconcentration of polymerized material 70 contacts the pore 95. Averagepolymerized material concentration location 85 can be located from about5 percent of the thickness extending from the first major surface 50 tothe second major surface 60. In some embodiments, the polymerizedmaterial concentration location 85 can be at least about 10 percent, atleast about 25 percent, at least 50 percent or at least about 75 percentof the thickness of the porous substrate extending from the first majorsurface 50 to the second major surface 60.

In some embodiments, an asymmetric membrane can be formed using amultilayer structure wherein the porous substrate is coated with apolymerizable composition as previously described to provide a coatedporous substrate. A first layer can be positioned adjacent to the firstmajor surface of the coated porous substrate, and a second layer can bepositioned adjacent to the second major surface of the coated poroussubstrate to thereby form a multilayer structure. The first layer andthe second layer may be discrete pieces of materials or they maycomprise continuous sheets of materials. On a continuous process line,for example, the first layer and the second layer may be unwound fromrolls and brought into contact with the coated porous substrate. Inforegoing embodiments wherein the coated porous substrate is positioned(i.e., sandwiched) between a first layer and a second layer to form amultilayer structure, a single roller or multiple rollers may be used tometer or remove excess polymerizable composition and entrapped airbubbles from the coated porous substrate. The first layer and the secondlayer of the multilayer structure may comprise any inert material thatis capable of providing temporary protection to the membrane fromexposure to oxygen upon exiting the ultraviolet radiation source havinga peak emission wavelength of less than 340 nm. Suitable materials forthe first layer and the second layer include, for example, sheetmaterials selected from polyethylene terephthalate (PET), biaxiallyoriented polypropylene (BOPP), fluorinate polyolefin available from 3MCompany and Dupont, other aromatic polymer film materials, and any othernon-reactive polymer film material. The first layer should besubstantially transparent to the peak emission wavelength of theultraviolet radiation source selected. Once assembled, the multilayerstructure typically proceeds to irradiation by the ultraviolet radiationsource. After irradiation, the first layer and the second layer can beremoved (i.e., eliminated) from the multilayer structure to provide theasymmetric membrane.

The thickness of the first layer of the multilayer structure cangenerally be in a range of 10 micrometers to 250 micrometers, 20micrometers to 200 micrometers, 25 micrometers to 175 micrometers, or 25micrometers to 150 micrometers. The second layer may have the same or adifferent thickness than that of the first layer. The first layer may bethe same material or a different material that that used for the secondlayer.

In some embodiments, a first layer is positioned adjacent to the firstmajor surface on the coated porous substrate to form a bi-layerstructure. The first layer can be positioned between the ultravioletradiation source and the coated porous substrate. After irradiation bythe ultraviolet radiation source, the first layer can be removed (i.e.,eliminated) from the bi-layer structure to provide the asymmetricmembrane.

In another embodiment, the coated porous substrate is free of a firstlayer and a second layer. The coated porous substrate may be subjectedto an inert atmosphere (e.g., nitrogen, argon) to reduce the penetrationof oxygen (e.g., provide an oxygen free environment) to the coatedporous substrate.

In some embodiments, the penetration of the ultraviolet radiation sourcecan be limited by the selection of the ultraviolet radiation sourcethrough the coated porous substrate to produce a gradient of polymerizedmaterial within the asymmetric membrane that can result in differentpolymerized material compositions on the first major surface and thesecond major surface. In some embodiments, polymerized material canreside on the first major surface and within a portion of the thicknessof the porous substrate. The polymerized material residing within thethickness of the porous substrate can have a gradient concentration ofpolymerized material extending from the first major surface to thesecond major surface. In one embodiment, an asymmetric membrane has ahydrophilic surface and a hydrophobic surface.

The asymmetric membrane formed by the method of the present disclosurecan have a variety of surface properties and structural characteristicsdepending on a number of factors. These factors include withoutlimitation the physical and chemical properties of the porous substrate,the geometry of the pores of the porous substrate (i.e., symmetric orasymmetric), the method of forming the porous substrate, the polymericspecies polymerized and retained as polymerized material with thesurfaces (i.e., first major, interstitial pore, and second major) of thecoated porous substrate, optional post-polymerization treatments (e.g.,a heating step) administered to the asymmetric membrane, and optionalpost-polymerization reactions (e.g., reactions of the additionalfunctional group of the polymerizable species species with a compoundsuch as a nucleophilic compound or a compound with an ionic group).

Asymmetric membranes of the present disclosure can exhibit variousdegrees of wettability upon exposure to various polymerizablecompositions. Wettability can often be correlated to the hydrophilic orhydrophobic character of the asymmetric membrane. As used herein, theterm “instant wet” or “instant wettability” refers to the penetration ofdroplets of water into a given asymmetric membrane as soon as the watercontacts the porous substrate surface, typically within less than 1second. For example, a surface wetting energy of about 72 dynes/cm orlarger usually results in instant wetting. As used herein, the term “noinstant wet” refers to penetration of droplets of water into a givensubstrate but not as soon as the water contacts the substrate surface.As used herein, the term “no wetting” refers to the lack of penetrationof droplets of water into a given asymmetric membrane. For example, asurface wetting energy of about 60 dynes/cm or less usually results inno wetting without applied pressure.

Application of polymerizable compositions onto a hydrophobic poroussubstrate and treating the coated hydrophobic porous substrate to theultraviolet radiation can result in a membrane having first and secondmajor surfaces having hydrophobic character, a first major surfacehaving hydrophilic character and a second major surface havinghydrophobic character, or first and second major surfaces havinghydrophilic character. Similarly, applying polymerizable species onto ahydrophilic porous substrate and treating the coated hydrophilic poroussubstrate to ultraviolet radiation can result in an asymmetric membranehaving first and second major surfaces having hydrophilic character, afirst major surface having hydrophobic character and a second majorsurface having hydrophilic character, or first and second major surfaceshaving hydrophobic character.

In one embodiment, the porous substrate is hydrophobic or hydrophilic.In another embodiment, an asymmetric membrane can be formed comprising ahydrophobic surface and a hydrophilic surface. A first major surface canbe hydrophilic and a second major surface can be hydrophobic.

In one embodiment, an asymmetric membrane can comprise a symmetricporous substrate. The asymmetric membrane can comprise a gradientconcentration of polymerized material extending from the first majorsurface to the second major surface, such that the concentration ofpolymerized material is greater at the first major surface than at thesecond major surface. In another embodiment, a first major surface ishydrophilic and a second major surface is hydrophobic.

In another embodiment, an asymmetric membrane can comprise an asymmetricporous substrate. The asymmetric membrane can comprise a gradientconcentration of polymerized material extending from the first majorsurface to the second major surface, such that the concentration ofpolymerized material is greater at the first major surface than at thesecond major surface. In another embodiment, a first major surface ishydrophilic and a second major surface is hydrophobic.

In one embodiment, the asymmetric membrane can be chemically asymmetric.The asymmetric membrane comprises a symmetric porous substrate having afirst major surface and a second major surface, wherein the majorsurfaces (e.g., being hydrophilic) can contain polymerized materialretained throughout at least a portion of the thickness of the poroussubstrate. The asymmetric membrane can have a greater concentration ofpolymerized material at the first major surface than at the second majorsurface.

In another embodiment, the asymmetric membrane can be physicallyasymmetric. For example, the physically asymmetric porous substrate canhave a greater concentration of the polymerized material at the firstmajor surface than at the second major surface. In some embodiments, thegradient of polymerized material can contribute to at least partiallyblocking of the pores on at least one major surface and an increasedpore size extending through the thickness of the porous substrate to asecond major surface.

Asymmetric membranes formed having a greater concentration ofpolymerized material at the first major surface than at the second majorsurface are described. Asymmetric membranes can find applications inwater softening, filtration, and chromatography. Asymmetric membranesformed by a continuous process provide for producing membranes moreefficiently and more economically.

The disclosure will be further clarified by the following examples whichare exemplary and not intended to limit the scope of the disclosure.

EXAMPLES

The present disclosure is more particularly described in the followingnon-limiting examples. Unless otherwise noted, all parts, percentages,and ratios reported in the following examples are on a weight basis.

Test Procedures Water Flux Measurements and MgCl₂ Rejection Measurements

Water flux and MgCl₂ (magnesium chloride, salt) rejection measurementsof the asymmetric membrane prepared above were measured with a stirredultrafiltration cell (model 8400; Millipore Corporation, Bedford, Mass.)having an active surface area of 41.8 cm². The trans-membrane pressurewas set at 50 psi (pounds per square inch) under pressurized nitrogengas. Water flux was calculated based upon the amount of water passingthrough the membrane as a function of time, asymmetric membrane area,and the set pressure. The MgCl₂ rejection (salt rejection) was obtainedfrom the conductivities of the permeate (C_(r)) and the feed (C_(f))(500 ppm MgCl₂ aqueous solution) according to the following equation;

${R( {MgCl}_{2} )} = {( {1 - \frac{C_{p}}{C_{f}}} ) \times 100\%}$

R=percent salt rejection.

The conductivity (C_(p) and C_(f)) was measured with a conductivitymeter (VWR Digital Conductivity Bench Meter; VWR International, WestChester, Pa.), and the mass of permeate was measured by an electronicbalance (model TE3102S; Sartorius, Edgewood, N.Y.). The conductivity andthe mass of the permeate data were collected as a function of time usingWinwedge 32 computer software (TAI Technologies, Philadelphia, Pa.).Measurements were discontinued after the salt rejection measurementsstarted to decline after reaching a plateau. The salt rejection wasadjusted by the feed concentration at the end of testing.

Asymmetric Membrane Process

Asymmetric membranes were prepared by a continuous process. Apolypropylene thermally induced phase separation (TIPS) membrane asdescribed in U.S. Pat. No. 4,726,989 (Mrozinski) was die-coated with apolymerizable composition to form a coated porous substrate. The coatedporous substrate was laminated between two liners in a gap-controllednip. One of the two liners (e.g., films) was laminated to the firstmajor surface and the other liner was laminated to the second majorsurface forming a multilayer structure. The biaxially orientedpolypropylene liners ((BOPP) films of 1.18 mil (30 micrometer)thickness; 3M Company, St. Paul, Minn.) had a transmittance of about78.5 percent (UVC) and 85.9 percent (UVA). The edges of the multilayerstructure (i.e. edges of the two liners) were sealed with a pressuresensitive adhesive tape (Scotch ATG Tape 926; 3M, St. Paul, Minn.). Themultilayer structure was enveloped by the BOPP liners and the excesspolymerizable composition on the coated porous substrate was minimized.The multilayer structure was irradiated with a Quantum MicrowaveMulti-Lamp UV Curing System having a 47″ long UV window (Model:Quant-23/48R, Quantum Technologies; Irvine, Calif.). The Quantum UVSystem used either UVA lamps (26169-3, UV A 365 nm Peak Lamps TL60/10R,Philips, Somerset, N.J.) or UVC lamps (23596-0, Germicidal Sterlilamp254 nm Lamps TUV115W, Philips, Somerset, N.J.). The line speed wasadjusted using the machine speed display. The intensity of theultraviolet radiation source was measured by a PowerMap radiometer (EITUV Power MAP Spectral Response, UV: A, B, C, V, Range: Low, Head S/N1408, Body S/N 1022; Sterling, Va.) as the multilayer structure wascarried through the UV tray. The polymerizable species of thepolymerizable composition were polymerized forming polymerized materialretained within the porous substrate. The multilayer substrate wascollected on a roll and the liners were removed. An asymmetric membranewas recovered. The asymmetric membrane was washed with distilled waterprior to further testing.

Example 1

A polypropylene microporous TIPS membrane (bubble point porediameter=0.58 μm; thickness of about 3.5-3.6 mil (85-95 micrometers),water flux of 10521/m²h·psi) was die coated with a polymerizablecomposition. The polymerizable composition comprised 3-acrylamidopropyltrimethyl ammonium chloride ((APTAC), 75 wt. % in water; Sigma Aldrich,St. Louis, Mo.); N,N′-methylenebisacylamide (97%; Alfa Aesar, Ward Hill,Mass.), and1-[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one(Irgacure 2959; Ciba Specialty Chemicals, Tarrytown, N.Y.) in anethanol/water solvent mixture (60/40 volume:volume ratio). The APTACconcentration was 0.55 mol/kg in the ethanol/water mixture.N,N′-methylenebisacylamide in ethanol/water and Irgacure 2959 had aconcentration of 10 mole percent and 2 mole percent, respectively,relative to APTAC of the polymerizable species. No pretreatment wasrequired for the porous substrate. The polymerizable composition wasapplied to the polypropylene microporous TIPS membrane for forming acoated porous substrate. The coated porous substrate was prepared asdescribed by Asymmetric Membrane Process prior to forming a multilayerstructure, and prior to irradiation by the UV radiation source. Themultilayer structure was conveyed by a continuous process apparatus at aline speed of about 30.5 cm/minute. The first major surface (side A) ofthe coated membrane was irradiated by a UVC radiation source (lightintensity of 5.77 mW/cm²). The wet membrane was about 110-120micrometers (4.4-4.6 mils) in thickness. The separation performance ofthe membrane is listed in Table 1.

Comparative Example 1 CE 1

Comparative Example 1 was prepared similarly to Example 1 except thatthe UV radiation source (UVA) used to irradiate one side of themembrane. The first major surface (side A) of the coated membrane wasirradiated by UVA (light intensity of 28.55 mW/cm²). The wet membranewas about 110-120 micrometers (4.4-4.6 mils) in thickness. Theseparation performance of the membrane is listed in Table 1.

TABLE 1 Light Side Pure 500 ppm 500 ppm Irradiation intensity facingwater flux MgCl₂ Flux MgCl₂ Rejection Membrane Source (mW/cm²) feed(kg/m²-h-psi) (kg/m²-h-psi) (%) Example 1 UVC 5.77 A 0.84 0.76 93.3 B0.80 0.71 49.9 CE 1 UVA 28.55 A 0.25 0.24 94.8 B 0.25 0.23 93.3

As illustrated in Table 1, Example 1 showed a 40% salt rejection changefrom Side A to Side B suggesting a gradient concentration of polymerizedmaterial extending from Side A to Side B. When Side B faced the feed,the salt rejection was reduced suggesting an asymmetric concentration ofthe polymerized material in the membrane. Comparative Example 1 showed anegligible percent salt rejection change from Side A to Side Bsuggesting a similar concentration of polymerized material extendingthrough the thickness of the membrane from Side A to Side B.

Example 1 irradiated with a UVC radiation source shows about a threefold improvement in pure water flux as compared to Comparative Example 1irradiated with a UVA radiation source.

Examples 2 and Comparative Example 2 CE 2

Example 2 (membrane of Example 1) and Comparative Example 2 (membrane ofComparative Example 1) were individually stained with a negative chargeddye under the trade designation METANIL YELLOW commercially availableform Alfa Aesar of Heysham, Lancashire, England. The membranes ofExample 2 and Comparative Example 2 were immersed into an aqueous dyesolution in a vial and stirred for about 24 hours. The membranes wereremoved from the vials, rinsed with deionized water and dried. Side Arepresented the first major surface and Side B represented the secondmajor surface of the membranes of Example 2 and Comparative Example 2.Table 2 lists the results.

TABLE 2 Side A (first major Side B (second major Wettable PolymerizableDye charge surface) dye binding surface) dye binding Side MembraneSpecies (+/−) (yes or no/color) (yes or no/color) (yes/no) Example 2APTAC (+) METANIL Yes/dark yellow No/organic yellow Side A (yes) YELLOW(−) Side B (no) CE 2 APTAC (+) METANIL Yes/light orange Yes/light orangeSide A (yes) YELLOW (−) Side B (yes)

As illustrated in Table 2, Example 2 showed an affinity for the yellowdye of the membrane at side A resulting in a dark yellow color, whereasthe yellow dye was washed away from Side B. The surface at Side A has agreater concentration of polymerized material than at Side B indicativeof an asymmetric membrane. Comparative Example 2 showed an affinity forthe yellow dye of the membrane nearly equivalent at both Side A and SideB. Each side of Comparative Example 2 had a light orange colorsuggesting a similar concentration of polymerized material at Side A andSide B indicating the formation of a symmetric membrane.

Examples 3-4

A polypropylene microporous TIPS membrane (bubble point porediameter=0.72 μm; thickness of 4.2-4.3 mil (120-130 micrometers), waterflux of 1475 l/m² h psi was die coated with the polymerizable species ofExample 1. No pretreatment of the polypropylene membrane was required.The polymerizable species was applied to the polypropylene microporousTIPS membrane to form a coated porous substrate. The coated poroussubstrate was prepared as described by Asymmetric Membrane Process priorto forming a multilayer structure and irradiating by the UVC radiationsource. The multilayer structure was conveyed on by a continuous processapparatus at a line speed of about 50 cm/minute. The first major surface(side A) of the coated membrane was irradiated with different UVC lightintensities (light intensity as measured by a PowerMap radiometer). Theseparation performance of the membranes of Examples 3-4 is shown inTable 3.

TABLE 3 Wet Pure water 500 ppm 500 ppm Light membrane flux MgCl₂ FluxMgCl₂ intensity thickness (kg/ (kg/ Rejection Membrane (mW/cm²) (μm)m²-h-psi) m²-h-psi) (%) Example 3 2.12 130-140 0.42 0.39 95.0% Example 45.77 120-128 0.95 0.84 91.9%

Table 3 illustrates the effects of UVC radiation at different lightintensity on coated porous substrates. A change in pure water flux canbe observed at higher UVC light intensities.

Examples 5-13

A polypropylene microporous TIPS membrane as used in Examples 3-4 wasdie coated with the polymerizable composition of Example 1. Nopretreatment of the PP membrane was required. The polymerizable specieswas applied to the polypropylene microporous TIPS membrane to form acoated porous substrate. The coated porous substrate was prepared asdescribed by the Asymmetric Membrane Process prior to forming amultilayer structure, and prior to irradiation by the UVC radiationsource. The multilayer structure was conveyed on a continuous processapparatus at variable line speeds shown in Table 4. The first majorsurface (side A) of the coated membrane was irradiated with differentUVC light intensities (light intensity as measured by a PowerMapradiometer). The separation performance of the membranes of Examples5-13 is listed in Table 4.

TABLE 4 Line Pure water 500 ppm 500 ppm Light Speed flux MgCl₂ FluxMgCl₂ intensity (cm/ (kg/ (kg/ Rejection Membrane (mW/cm²) minute)m²-h-psi) m²-h-psi) (%) Example 5 2.1 30.5 0.50 0.45 94.6 Example 6 2.1152.4 0.61 0.56 93.9 Example 7 2.1 243.8 0.59 0.54 94.4 Example 8 3.930.5 0.76 0.68 92.8 Example 9 3.9 152.4 1.03 0.92 90.1 Example 10 3.9243.8 1.17 1.06 89.3 Example 11 5.7 30.5 1.08 0.96 90.6 Example 12 5.7152.4 1.50 1.32 85.8 Example 13 5.7 243.8 1.90 1.71 80.4

Table 4 illustrates the effect of line speed and UVC light intensity oncoated porous substrates for forming membranes.

Various modifications and alterations of this disclosure will beapparent to those skilled in the art without departing from the scopeand spirit of this disclosure, and it should be understood that thisdisclosure is not limited to the illustrative elements set forth herein.

What is claimed is:
 1. An asymmetric membrane formed by a methodcomprising: providing a porous substrate having a first major surfaceand a second major surface; applying a polymerizable composition to theporous substrate providing a coated porous substrate, the polymerizablecomposition comprising i) at least one polymerizable species; and ii) atleast one photoinitiator; and exposing the coated porous substrate to anultraviolet radiation source having a peak emission wavelength less than340 nm to polymerize the polymerizable species, wherein thephotoinitiator residing at the first major surface is exposed to agreater peak irradiance of ultraviolet radiation than the photoinitiatorresiding further into the thickness of the porous substrate thusreducing the amount of polymerization within the pores of the substratethrough the thickness of the substrate, thereby providing an asymmetricmembrane, the asymmetric membrane having a polymerized material retainedwithin the porous substrate, the polymerized material having aconcentration greater at the first major surface than at the secondmajor surface.
 2. The asymmetric membrane of claim 1, whereinun-polymerized material within the porous substrate is removed.
 3. Theasymmetric membrane of claim 1, wherein the porous substrate ismicroporous.
 4. The asymmetric membrane of claim 1, having a gradientconcentration of polymerized polymerizable material extending from thefirst major surface to the second major surface.
 5. The asymmetricmembrane of claim 1, wherein the porous substrate is microporous.
 6. Theasymmetric membrane of claim 1, wherein the porous substrate comprises amicroporous, thermally-induced phase separation membrane.
 7. Theasymmetric membrane of claim 1, wherein the porous substrate ishydrophilic.
 8. The asymmetric membrane of claim 1, wherein the poroussubstrate is hydrophobic.
 9. The asymmetric membrane of claim 1, whereinthe porous substrate comprises a film, a nonwoven web, a woven web, afiber, or combinations thereof.
 10. The asymmetric membrane of claim 9,wherein the porous substrate further comprises a particulate.
 11. Theasymmetric membrane of claim 9, wherein the fiber is a hollow fiber. 12.The asymmetric membrane of claim 1, wherein the porous substratecomprises polyolefins, polyamides, fluorinated polymers,poly(ether)sulfones, cellulosics, poly(ether)imides, polyacrylonitriles,polyvinyl chlorides, ceramics, or combinations thereof.
 13. Theasymmetric membrane of claim 12, wherein the porous substrate comprisespolyolefins.
 14. The asymmetric membrane of claim 13, wherein thepolyolefins comprise polyethylene or polypropylene.
 15. The asymmetricmembrane of claim 12, wherein the porous substrate comprises polyamides.16. The asymmetric membrane of claim 1, wherein at least one of thepolymerizable species comprises acrylates, (meth)acrylates,(meth)acrylamides, styrenics, allylics, vinyl ethers, or combinationsthereof.
 17. The asymmetric membrane of claim 1, wherein at least one ofthe polymerizable species comprises an ionic group.
 18. The asymmetricmembrane of claim 17, wherein the ionic group comprises an amine or aquaternary ammonium salt.
 19. The asymmetric membrane of claim 17,wherein the ionic group comprises a carboxylic acid or a carboxylic acidsalt.
 20. The asymmetric membrane of claim 1, wherein the first majorsurface of the asymmetric membrane is hydrophilic, and the second majorsurface of the asymmetric membrane is hydrophobic.